This invention relates generally to JFET operational amplifiers. More specifically, the present invention provides circuits and methods for extending the input common mode voltage range of JFET operational amplifiers.
Operational amplifiers are among the most useful and cost-effective analog electronic devices. With only a handful of external components, operational amplifiers (xe2x80x9cop-ampsxe2x80x9d) can be used to perform a variety of mathematical and signal processing operations, including the addition, subtraction, integration, differentiation, and filtering of electronic signals. Depending on the application, op-amps may be selected for speed, noise, output power, and power dissipation. There are many types of op-amps currently available, with each type optimizing different performance parameters.
Modern op-amps are typically designed as linear integrated circuits consisting of three stages: an input stage with a differential input and a differential output, a second or intermediate stage with a differential input and a single output, and an output stage, which usually has unity voltage gain. The input stage amplifies the input voltage difference, the second stage provides frequency compensation, and the output stage provides output drive capability. There are many possible design variations on the structure of the three stages, with the main variations occurring on the devices used in the input stage. The input stage can be built using bipolar transistors (BJTs) for low voltage offset and low voltage noise, MOSFETS for low power operation, or JFETs for high input impedance, low bias current, and low current noise.
The three stages are commonly represented by a single schematic symbol, having two input terminals, referred to as the inverting and non-inverting inputs, an output terminal, and two DC power supplies. For illustrative purposes, a schematic symbol of an op-amp is shown in FIG. 1. The output voltage Vo is simply the difference in voltage between the non-inverting input voltage Vp and the inverting input voltage Vn, multiplied by the gain. The differential input and output voltages may be measured with respect to the input common mode voltage, which is a voltage that is common to the inverting and non-inverting inputs of the op-amp. The use of dual power supplies allows the input and output voltages to swing both positive and negative with respect to the input common mode voltage, which acts as the zero reference point.
Ideally, if the two inputs were to be shorted together with an input common mode voltage, thus ensuring zero voltage difference between them, there should be no change in the output voltage. That is, as the input common mode voltage is varied, the output voltage should hold absolutely steady. In practice, however, this is not easily attained. The input common mode voltage will invariably have some effect on the op-amp""s output voltage.
The performance of a real op-amp in this regard is most commonly measured in terms of its input common mode voltage range, which is the range of the input common mode voltage over which the op-amp is guaranteed to operate as a linear amplifier, and the common mode rejection ratio (CMRR), which is a measure of the ability of the op-amp to reject common-mode signals, i.e., signals that are simultaneously present at both inputs. The function of an op-amp is to amplify only the differential portion of the input voltages while rejecting the input common mode voltage. If the input common mode voltage varies beyond the input common mode voltage range, the inputs will shut down and proper operation ceases, i.e., the op-amp will be saturated.
The input common mode voltage range is an op-amp characteristic that depends on several factors, including the design used for the input stage. For example, the bipolar-based LM741 op-amp manufactured by National Semiconductor Corporation, of Santa Clara, Calif., and the LT1124 op-amp manufactured by Linear Technology Corporation, of Milpitas, Calif., have a guaranteed input common mode voltage range of +/xe2x88x9212 Volts for a supply voltage of +/xe2x88x9215 Volts, while the FET-based LT1113 manufactured by Linear Technology Corporation, of Milpitas, Calif., has a guaranteed input common mode voltage range of xe2x88x9210.5 to 13 Volts for a supply voltage of +/xe2x88x9215 Volts. The higher the input common mode voltage range of the op-amp, the longer the op-amp stays in the linear operating region.
Increasing the input common mode voltage range of an op-amp, however, often comes at the sacrifice of other performance characteristics, such as offset voltage, offset drift, and noise. In particular, JFET op-amps may suffer from phase inversion if the input common mode voltage approaches either supply too closely. When this occurs, the inverting and non-inverting input terminals reverse functions and the output may reverse direction.
In view of the foregoing, it would be desirable to provide circuits and methods for extending the input common mode voltage range of JFET op-amps.
It also would be desirable to provide circuits and methods for extending the input common mode voltage range of JFET op-amps while avoiding phase inversion, increase in input bias current, and changes in output polarity.
In view of the foregoing, it is an object of the present invention to provide circuits and methods for extending the input common mode voltage range of JFET op-amps.
It also is an object of the present invention to provide circuits and methods for extending the input common mode voltage range of JFET op-amps while avoiding phase inversion, increase in input bias current, and changes in output polarity.
These and other objects of the present invention are accomplished by providing circuits and methods for extending the input common mode voltage range of a JFET op-amp by modifying the input stage of the JFET op-amp. In a preferred embodiment, the input stage of the JFET op-amp is modified to include a BJT pair as the input differential pair and use the JFET pair as followers. Using the JFETs as followers enables the BJT differential pair to remain in the same region of operation over the entire common mode voltage range. The negative common mode voltage range increases when the JFET followers change from the saturation region of operation to the linear region of operation. The positive common mode voltage range is increased by reducing the source current in the JFET followers and using an additional transistor pair as clamping transistors.
Advantageously, the present invention enables a JFET op-amp to have an extended input common mode voltage range while keeping the bandwidth constant and avoiding phase inversion and other performance drawbacks commonly associated with a wider common mode voltage range.